Distortion requires opposing forces
Definition
Elastic distortion
A change in shape that is reversed when the forces causing it are removed.
Definition
Inelastic distortion
A permanent change in shape that remains after the forces causing it are removed.
- A single force accelerates an unsupported object, whereas distortion requires forces acting at different points or in different directions.
- Pulling both ends of a spring stretches it, pushing both ends compresses it, and applying separated forces can bend an object.
- The forces do not have to be equal while the shape is changing, but both are needed to produce stretching, compression or bending.
Elastic and inelastic changes differ
- After elastic distortion, the object returns to its original length and shape when the forces are removed.
- After inelastic distortion, some or all of the change remains because the object has been permanently deformed.
- A spring can behave elastically for small forces but become inelastically distorted if it is overloaded.
Common Mistake
Do not describe an object as elastic merely because it stretches; it must return to its original shape after the forces are removed.
Example
Classifying distortion
- A spring has an original length of 0.120 m0.120\,\text{m}0.120m, reaches 0.165 m0.165\,\text{m}0.165m under load and returns to 0.120 m0.120\,\text{m}0.120m when unloaded, so the distortion is elastic.
- If it returns only to 0.128 m0.128\,\text{m}0.128m, the remaining extension is permanent and the distortion is inelastic.
Self review
- Why are at least two forces needed to distort an object?
- What is elastic distortion?
- How can you identify inelastic distortion after removing the force?
- Why can the same spring show elastic and inelastic behaviour under different loads?
